Wastewater in this application refers to any aqueous fluid that without prior treatment is not suitable for human consumption or industry application or discharge from any facility because of the existence of natural or artificial contaminants. The contaminants include organics, particulates, sub-micro particles, microorganisms such as viruses and bacteria, and dissolved metals. Wastewater is being continuously generated by nature (e.g., storm, mudslides, animals, and growth of microorganisms) and human activities (e.g., domestic consumption, and industry applications); it imposes a grave challenge to provide suitable water supply for human consumption and industry applications because of limited water reservoir on the Earth. Therefore, wastewater treatment is critical for provision of reusable water and limit of spreading of contamination from untreated discharge from wastewater-generating industries.
Electrolysis process (often referred as electrocoagulation) has been proven to be able to treat a variety of wastewater including paper pulp mill waste, metal plating, tanneries, caning factories, steel mill effluent, slaughterhouses, chromate, lead and mercury-laden effluents, domestic sewage, and radioactive materials. It has the capability of removing a large range of contaminants under a variety of conditions ranging from: suspended solids, heavy metals; petroleum products, color from dye-containing solution, aquatic humus, and defluoridation of water. The treatment provides clear, clean, odorless and reusable water.
Electrocoagulation is a complex process with a multitude of mechanisms operating synergistically to remove contaminants from wastewater. Electrocoagulation employs a pair of electrodes to neutralize small charged particles in colloidal suspension. The electrodes are usually made of aluminum or iron. When the electrodes (anode and cathode) are subjected to a specific current density, the anodes are oxidized and form metal ions (either Fe+2, Fe+ or Al+3) in solution that react with hydroxide (OH—) anions created in the electrocoagulation process. This leads to the formation of metal hydroxide ions, either cationic or anionic species depending on the pH of the wastewater. A combination of inert anodes and metal (titanium) cathodes can also be used. The inert electrodes accomplish contaminant destabilization utilizing the transfer of electrons within the electrolyte. The transfer of electrons and formation of protons (H+) created in the electrocoagulation process can effectively destabilize a range of metal and organic contaminant species.
A typical electrocoagulation reactor contains a series of substantially parallel electrolytic plates or electrodes through which the wastewater to be treated travels in a serpentine path while being exposed to a strong electric field or voltage. For over the past twenty years, in order to try to find a more environmentally friendly way to treat wastewater, many electrocoagulation (EC) systems were designed and built for many wastewater treatment applications. For example, U.S. Pat. No. 6,689,271 discloses an apparatus for electrocoagulation treatment of industrial wastewater. However, a broad use of the EC systems is limited by unsolved technical obstacles.
The main technical obstacles affecting the efficiency and performance of EC devices include the corrosion and passivation of electrodes and the accumulation of gases in an EC device. Electrodes are easily coated with contaminants, corroded and oxidized by wastewater, thus unable to evenly distribute the ion density in wastewater. Therefore, regular cleaning and replacement of electrodes were normally required. In addition, the oxygen and hydrogen gases are gathered over time at the electrodes and not utilized fully for treating the wastewater, causing a reduction or stoppage of electrolysis action after some time. These result in higher electrical power consumption than expected, slower separation of flocculants from the water at the output, higher percentage of sludge and lower percentage of floating flocculants due to inefficient use of hydrogen gas, and required post-treatment of sludge.
Attempts have been made to address the problem of passivation of electrodes during the electrocoagulation process by constructing self-cleaning electrolytic cells. For example, US 2003/0222030 A1 discloses an electro-coagulation treatment system with an electrolytic cell including an anode and a helical cathode. It claims that the provision of a helical cathode in the form of a helically wound coil of a wire or rod of circular cross section provides an arrangement in which the cell is automatically self-cleaning in that the coagulated precipitates are carried from the cell by the flow of the water. However, the construction of such a helical cathode is a challenge and increases its cost. In addition, CN 01108767.6 discloses an EC device with a wiper to remove any deposits from the surfaces of electrodes. However, the wiper is in firm contact with surfaces of electrodes, and this causes unnecessary wearing out of the electrodes.
Attempts also have been made to reduce the sludge by increasing the flocculants. For example, U.S. Pat. No. 6,719,894 discloses an apparatus for treating organics, particulates and metal contaminates in a waste fluid. The apparatus has a pressurizing means for pressurizing waste fluid to be treated in the reactor vessel so that water, organics, particulates and metal contaminants form dissolved gases and form precipitate particles in the pressurized waste fluid. When the pressure of the treated waste fluid is reduced, dissolved gases evolve from the waste fluid causing said precipitate particles to float to a fluid surface for removal. However, the introduction of pressure complicates the system.